Conventional smoke detectors have particular disadvantages owing to the lack of reliability, expense and high current requirements of the electromechanical alarm horns currently used. These electromechanical horns have relatively high current demands for starting the alarm (about 300 milliamperes, ma) and sustaining operation at steady state (about 70 ma), which for direct current smoke detectors requires the use of expensive alkaline or mercury batteries for reasonable battery life. Accordingly, there is a need for a reliable horn with low current demands so that cheaper zinc/carbon or zinc chloride batteries may be used.
The realiability and low current demands of piezoelectric horns are well known in the prior art. However, such horns were limited, prior to this invention, to low audio output applications such as video games and electronic pinball machines. Such horns could not be used to meet the Underwriters Laboratories alarm requirement of 85 decibels (db) at 10 feet from the smoke detector without use of prohibitively high drive voltages and currents.
The use of detectors having two ionization chambers is also known. Generally, one chamber is almost completely closed to the atmosphere and is used to obtain a reference current for comparison to the current detected in the test chamber which is freely open to the surrounding air. When the two currents differ by more than a predetermined amount the alarm is triggered. In the past, problems have been encountered when these detectors were exposed to changes in humidity or pressure causing changes in the current measured in the test chamber but not in the current measured in the reference chamber. The difference in these currents often led to activation of the alarm when no smoke was present.
Smoke detectors are normally mounted on room ceilings so as to contact smoke where it first accumulates in the room. It is desirable to periodically test the detector to assure that the battery is sufficiently charged and circuits are operating, but this is difficult where the detector is attached to the ceiling and out of reach. Existing devices have small buttons or levers for activating the tester circuit which make testing even more inconvenient when the detector is out of reach.